1. Technical Field of the Invention
The present invention relates to an image forming apparatus for use in copying machines, printers and similar image forming apparatus, and more particularly, to an image forming apparatus capable of forming images on both sides of a transfer sheet by collectively transferring a visualized image formed on a photoconductor and another visualized image which had been formed on the photoconductor and transferred on an intermediate transfer medium onto a transfer sheet which passes through the location opposite to the photoconductor and the intermediate transfer medium.
2. Brief Description of Related Art
As a general method of forming images on both sides of a sheet in a conventional copying machine, it has heretofore been known that a first image formed on a photoconductor is transferred and fixed on one side of a transfer sheet and then the sheet on which the first image has been transferred is turned over for forming a second image whereby the second image is transferred and fixed on another side of the transfer sheet. In U.S. Pat. No. 3,697,170, there is disclosed an image forming apparatus which is arranged to collectively transfer images on both sides of a transfer sheet by using a photoconductor and transfer roller or transfer belt. Theoretically, either the transfer roller or the transfer belt may be used with the same effect.
In the case of an apparatus which is provided with the transfer belt, for instance, a first image formed on the photoconductor is transferred onto the transfer belt by a transfer charger and toner polarity of the first image transferred on the transfer belt is inverted by a toner polarity inversion charger. The first image on the transfer belt and the second image formed on the photoconductor are simultaneously transferred by the transfer charger on both sides of the transfer sheet fed to the location between the photoconductor and the transfer belt, and the images are formed on both sides of the sheet after fixing process.
In the method of separately transferring and fixing the first and second images, however, it is necessary to re-supply the transfer sheet on which the first image has been transferred and fixed for transferring and fixing the second image by turning over the transfer sheet. Accordingly, another sheet transport path is required for the re-supply of the transfer sheet thereby causing the size of the apparatus to be large, 30-40% larger in volume, resulting in a considerable rise in production cost.
Another problem is that heat curling can occur during a first round of a fixing process, and it causes another problem of an irregular sheet feed in the transport path in a second round of transfer and fixing operations. Moreover, considerable time is required for forming images on both sides of the sheet since two rounds of paper feed have to be performed.
The apparatus disclosed in U.S. Pat. No. 3,697,170 is capable of solving such problems as described above since images are simultaneously transferred on both sides of a transfer sheet. However, there is a difficulty in making sufficient polarity inversion since toner polarity of a first image on the transfer belt is inverted, and an electric charge to be applied to toner becomes small.
Generally, a film or elastic belt material is used as a material for the transfer belt. If a thin film is used repeatedly, the edge of the film is easily broken, and in the case of an elastic belt, it is easily expanded to invite a problem of insufficient synchronization between the images. In order to cope with such problems, it is necessary to make the belt thick. However, if the belt is made too thick, electrostatic capacity C of toner layer and the transfer belt become small. Therefore, when polarity inversion is made by corona charge, sufficient amount of electric charge Q can not be given exceeding Q=CV (V is voltage). It may be considered to raise the voltage V, however, if the amount of electricity becomes sufficiently large, there occurs leakage through the transfer belt and a toner image is put into disorder. The transfer efficiency is thus lowered since a sufficient amount of electricity can not be given and the polarity of the whole toner can not be inverted. Dispersion of toner or the like easily occurs since the amount of electric charge of toner is small.
The apparatus is, therefore, easily affected by the quality and thickness of transfer sheet at the time of the transfer process thereby causing toner dispersion and a poor transfer of image. Moreover, when an image is formed on one side of a transfer sheet only, a proper image formation can not be achieved since transfer efficiency is different from the case of collective transfer on both sides of the sheet.